Method of purifying and desulfurizing zinc sulfide ores and concentrates



Aug. 12, 1958 c. c. LONG ETAL METHOD OF PURIFYING AND DESULFURIZING ZINC7 SULFIDE ORES AND CONCENTRATES Filed Jan.

2 Sheets-Sheet 1 0 536m 525.3% T CE: 22; M12586 A R ow mm @v on 536m Wmocfiafiwfi T 2w: 2.2; 4 0625%; .w x om INVENTORS o CARLETON c. 1.0m, &HERAND K. NAJARIAN Jyrw%7- d/uow ATTORNEYS Aug. 12, 1958 Filed Jan. '7,1952 METHOD OF PI'JRI SULFIDE CRES AND CONCENTRATES C. LONG ET ALPRECIPITATOR $122 is 9 64 65 FYING AND DESULFURIZING ZINC 2 Sheets-Sheet2 3 ELECTRICAL .fll ,-ss

9 9| ELECTRICAL 87 WASTE HEAT PRECIPITATOR F: BOILER 89 INVENTORSCARLETON C. LONG &

HERAND K. NAJARIAN ATTORNEYS 1 product metal.

United States Patent METHOD OF PURIFYING AND DESUL'FURIZING ZINC SULFIDEORES AND CONCENTRATES Application January 7, 1952, Serial No. 265,318

3 Claims. (Cl. 75-9) This invention relates to a method and apparatusfor purifying zinc sulphide ores and commercial zinc sulphideconcentrates, whereby there is attained a high degree of purification,and subsequently roasting the purified concentrates in a highlyeconomical manner. Briefly, this invention contemplates initialpurification of the concentrates by removal therefrom of the majorportion of such constituents as lead, cadmium, tin, and the like, butwith only partial oxidation of the sulphide constituents. Theso-purified concentrates, partially depleted of fuel value (i. e.,sulphide sulphur), are then in a subsequent and separate operationdesulphurized and oxidized by roasting in a fluidized bed. Both theinitial purification and the subsequent desulphurization are carried outessentially autogenously. The combination of purification and subsequentfinal desulphurization as separate steps carried on in substantiallyseparate apparatus but with both being achieved autogenously marks adistinct forward step in Zinc metallurgy and in the economy of zincsmelting.

In the art of zinc metallurgy, one of the most challenging problems isthe production of high grade low-lead zinc from a zinc sulphideconcentrate containing substantial amounts of lead. Zinc concentratescontaining less than 0.1% lead are much less commonly available thanthose concentrates containing 0.1% to 1.0% lead. Even more abundant arezinc concentrates containing more than 1% lead; and in fact, leadcontents as high as 3% are not unusual.

In the present state of the art, production of high grade zinc metal orzinc oxide requires either preselection of concentrate feed, wetpurification, or rectification of the Rectification of product metalinvolves fractional distillation of high-lead zinc; e. g., prime westerngrade This method, while technically satisfactory, inherently involvesadded cost of the extra metallurgical step. Wet purification by thewell-known electrolytic Zinc process is generally more costly than theprocesses of Zincpyrometallurgy and finds its most extensive use onlywhere the Zinc ores contain significant values in byproduct metals suchas gold and silver. Direct production ofhigh grade zinc metal bypyrometallurgical processes is achieved only when the lead content ofthe zinc concentrate feed is limited to what can be removed in theroasting and sintering operations. For example, in the well-knownelectrothermic method for zinc production, it is generally foundnecessary to limit the lead con when to do so results in lower recoveryof zinc, as happens in the present state of the art of beneficiation.Highleadzinc concentrates are used to produce prime western zinc forwhich there is a large and steady demand. There is also a large andincreasing demand for high grade zinc. In view of the ready outlet forhigh-lead zinc concentrates, there appears little possibility ofsubstantially improving the supply of low-lead zinc concentrates at themines. The producers of zinc metal must, therefore, develop moreefficient and lower cost purification procesess.

An object of this invention is to provide a method and apparatus wherebya relatively pure high grade zinc calcine may be made at reasonable costfrom a high-lead relatively impure zinc sulphide concentrate.

Another object of this invention is to provide a method and apparatusfor beneficiating a high-lead relatively impure zinc sulphideconcentrate whereby two products are produced; one a low-lead relativelypure zinc calcine suitable for the production of high grade zinc, theother a lead-enriched calcine suitable for the production of primewestern zinc.

Another object of this invention is to provide a method and apparatusfor beneficiating a high-lead relatively impure zinc sulphideconcentrate whereby two products or fractions are produced; one arelatively pure zinc calcine suitable for the production of high gradezinc Without further refining, the other fraction having a relativelyhigh concentration of impurities such as lead, cadmium, and so forth,the proportion of the second fraction being relatively small comparedwith the first fraction.

A further object of this invention is to provide a method and apparatuswhereby the above-described objects may be achieved by essentiallyautogenous means without necessity of combustion of substantial amountsof added fuel.

A still further object of'this invention is to produce as a by-productof this purification and desulphurization process a high-strengthsulphur dioxide gas suitable after purification and dilution with airfor manufacture of sulphuric acid by the contact process.

Another object of this invention is to provide a method and apparatusfor purifying and roasting zinc sulphide concentrates at an over-allcost substantially lower than that achieved by prior art methods.

The term zinc calcine as used in this specification is that commonlyemployed in the art to designate an essentially dead roasted material;i. e., the product obtained by substantially complete oxidation of azinc sulphide concentrate.

It is known that by subjecting the commercial zinc sulphide concentratescontaining relatively large amounts of impurities to an oxidizing roastin any of Well-known processes such as roasting in multiple hearthroasters, suspension-type roasters, rotating drum masters, and the like,appreciable amounts of volatile impurities such as lead, cadmium, andthe like, remain in the final roasted product, making the productunsuitable for production of high grade zinc metal or zinc oxidepigments without further purification.

By the method of the invention disclosed herein, lowlead calcinesuitable for the production of high grade zinc metal or zinc oxide canbe produced from high-lead zinc sulphide concentrates; i. e., zincconcentrates containing more than 0.5% lead. Briefly, the inventionconsists of obtaining purification and subsequent desulphurization intwo distinct steps, whereby in the first step the lead-containing zincsulphide material is subjected to a high temperature and a high sulphidesulphur environment wherein the major portion of the impurities iseliminated, and then, in the second step, passing the thuspurifiedmaterial through a fluid bed reactor for substantially complete removalof the sulphur by oxidation. Under these conditions, the lead-containingconstituents readily sublime and are carried oft-with the exit gasesfrom the first step or purification operation, while other minor metalswhose sulphides, oxides, or intermediate reaction products are volatile,such as cadmium and tin, are vaporized insubstantial amounts and'areremoved along with the lead,

In the usual method of carrying out the purification step, a portion ofthe original sulphide content of the concentrate feed is oxidized toprovide the necessary heat. The purified partially oxidized intermediateproductwhich, for nomenclatural convenience will hereinafter bedesignated as PDC, i. e., partially desulphurized concentrate-ordinarilydoes not contain enough heat value to permit completely autogenousroasting by the methods conventionally employed in the art. That is tosay, PDC cannot be dead roasted in a multiple hearth furnace, asuspension roaster, a rotary kiln, or the like, without the use ofsubstantial quantities of auxiliary fuel, such as natural gas. Theinvention described herein provides a method and apparatus bywhich theroasting of PDC may be completed entirely autogenously.

In one of the preferred methods of the invention, as the first step inpurification and desulphurization or roasting procedure, sulphidic zincconcentrates are fed to the top hearth of a conventional type multiplehearth roaster. The concentrates successively pass downwardly through amultiplicity of hearths while being mechanically agitated by means ofrabbles and being spread over several hearths and transported toward thebottom discharge opening while hot gases are circulated upwardly overthe several hearths. I

This method of operating such an assembly, whereby practically completeelimination of impurities such as lead and very high percentageelimination of cadmium and the like is obtained, differs from theconventional roasting of zinc concentrates in multiple hearth roastersas follows:

The top open hearth of the multiple hearth roaster is used mainly fordrying of the concentrates. Depending upon the capacity desired, thefirst, second, or third hearths following immediately below the tophearth are used to gradually heat the material to a temperaturepermitting ignition of sulphidic materials in the charge whereby whenthe material drops on to the next following hearth below the sulphidicminerals begin to ignite. This hearth and the several hearths followingare used as reaction hearths in which sufficient preheated oxygen-leangases of high sulphur dioxide content are allowed to circulate upwardlyover each one of the several reaction hearths partially oxidizing thesulphidic minerals to maintain a temperature and atmosphere in each ofseveral hearths which will permit volatilization of the volatilecompounds of metals such as lead, cadmium, and the like. As the productsof combustion, due to strict control of air supply, are maintained at ornear maximum possible concentration with respect to sulphur dioxidecontent, a preponderantly reducing atmosphere surrounds the materialbeing agitated and transported downwardly over the reaction hearths.Preferred temperature range for volatilization of impurities in the zincconcentrates and for incidental partial oxidation thereof as they passover the reaction hearths, is between 850 to 950 C. The lowest two orthree hearths in the multiple hearth furnace immediately below thelowest reaction hearth are used for the dual purpose of preheating thesmall amount of'air required to temperatures suflicient to oxidize theminerals in the lowest of the reaction hearths and, also, to cool thepurified material before its discharge from the roaster. The preheatedair may comprise hot gas from another furnace, such as a subsequentdesulphurization reactor or other desirable source, and may be soregulated as to be capable of burning sufficient sulphidic minerals inthe reaction hearths to maintain the desired temperature. Additional airor heated gas is supplied to each one of the reaction hearths necessaryto oxidize an increment of sulphidicminerals to maintain optimumoperating conditions.

When purifying commercial zinc concentrates having 30% to 32% sulphur,oxidation of somewhat less than one-half of the sulphur content of theconcentrates is usually found sufiicient to furnish the heat necessaryto complete the purification and partial desulphurization operation asoutlined hereinbefore during normal operations without the necessity ofsupplying extraneous heat. Assaysyof product discharged from themultiple hearth roaster show from 18% to 20% sulphur remaining in thepartially desulphurized product. Zinc ores and zinc concentratescontaining sulphur lower or higher than 30% to 32% can be purified bythis method.

7 centrates, however, a further improvement is employed.

' centrates fed to the furnace.

By thoroughly drying and preheating if necessary the concentrate beforeit is fed onto the top hearth of the multiple hearth furnace, thefurnace can be run with a hot top. By this is meant that the temperatureof the upper hearths of the furnace is kept sufiiciently high to preventcondensation and recycling of lead sulphide, lead oxide, and otherimpurities on and with the incoming ore. Without wishing to confine theinvention to a par-' ticular theory, it is thought that for very highlead content feeds the furnace gases in the upper part of the roastermay become supersaturated, with respect to constituents volatilized fromthe charge, if the temperature is allowed to drop too much. Presumablythe supersaturation is relieved by condensation from the gas of theexcess sublimed constituents. Some of the so formed particulate mattermay settle out on the incoming ore where it has the effect of increasingthe percentage of impurities in the charge.

Whether the above hypothesis be correct or not, the practical effect andthe remedy as described have been discovered.

The volatile impurities in the zinc concentrates that are driven off inthe reaction hearths together with very fine particles of zincconcentrates entrained in the gases from the multiple hearth roaster arecarried out of the furnace, and are recovered in well-known apparatusessuch as cyclones, electrical precipitators, and the like. A typicalanalysis of the fume and dust collected from the gases discharged fromthe multiple hearth furnaces which are used for purification and partialdesulphuriza tion shows lead assay of 18% to 20%, zinc assay of 38% to42%. The amount of solid fume material discharged with the furnace gasesfrom such an operation is in the neighborhood of 2.5% of the weight ofcon- When a multiple hearth roaster is being used in a conventionalmanner for dead roasting of commercial zinc concentrates, the amount ofdust and fume driven out of the furnace with the furnace gases usuallyruns from 12% to 15% of the weight of concentrates fed to the furnacewith a typical average assay of 4.5% to 5% lead and about 50% zinc. Itis evident that in the practice of the present invention, due to lowvolume of gases being circulated in the furnace and consequent lowvelocity and lower exist temperature, much less zinc concentrate isdriven out of the furnace as dust, and the dust collected in thecyclones or electric precipitators is much higher in lead content,making it more economical to further process the same for separation oflead, cadmium, zinc, etc.

As the second step in the purification and roasting of commercial zincconcentrates in accordance with this method, the partially roasted andpurified product from the multiple hearth furnace is passed throughanother furnace where complete desulphurization is obtained.

To obtain autogenous roasting of concentrates, substan-' tial savings infuel and economical operation, it is preferred to carry on the secondstep in a fluid bed furnace.

5 arrangcdso thatasubstantial depth of the PDC is maintained upon thefurnace hearth and so that the air'required to complete the oxidation isso injected as to obtain the agitationof the PDC being oxidized. Amixture of gases such as S; and air or S0 and oxygen may also be thevehicle of agitation.

It is well-known in the art of zinc metallurgy that a certain amount oflead is removed from zinc sulphide concentrates by .usual roasting understrongly oxidizing conditions in multiple hearth furnaces undercarefully controlled conditions. In the pyrometallurgical zinc process,for example, lead content of the product metal is influencedfin majordegree'by'the amount of lead eliminated during roasting. With goodpractice, it ispossible to .eliminate about 90% of the lead during theusual roasting in the multiple hearth furnace. For example, aconcentratecontaining 0.5 lead' can be made to produce a calcinecontaining about 0.05% lead. From a concentrate' containing 1%flead, acalcine containing 0.1% to 0.15% lead can'befproduced. v

In the subsequent operation 'of sintering, additionalbut lessimportant'amounts of lead are removed, mak ing it possible to producefromthe smelting furnaces a high .grade metalcontaining as low as 0.025%lead when the feed to the roaster contains as much as 0.5% lead.

Whereas prior art processes attempt to remove lead. by roasting; i. e.,'oxidation, this invention makes'it possible to removelead and similarimpurities more efiectively by minimizing oxidation and maintaining ashigh a concentration of sulphides as possible. 1 In this processthere ispractically no free oxygen in the exit gases from the first stage orpurification reactor, quite in contrast to prior art processes in whichthe exit gases commonly contain 6% to 71/2% sulphur dioxide and 7 to 10%oxygen. In the process of this invention, the sulphur dioxide contentofthe exit gases approximates 14% to 16%, depending on the composition ofthe air or other fluidizing gases entering the reactor.

The process is preferably operated continuously with high-lead zincconcentrate feed entering the purification reactor at a predeterminedrate and deleaded PDC being withdrawn from the reactor at a relatedrate. Heat is normally supplied by admitting air to the reactor andpartially burning the concentrates.

Whena multiple hearth roaster is operated according to this method 'forinitial purification and partial desulphurizationof commercial zincconcentrates, much larger tonnages canbe treated in a given size offurnace than when the same "furnace is used for roasting inconventionalmanner.

In a typical run of the purification step, zinc sulphide concentratesare'fe'd' at the rate of 132 tons'per day to a Nichols-Herreshoiffurnace-20'"6 inside diameter, with twelve super-imposed'hearths andhaving anormal oxidizing or dead roasting capacity of '60 to 70 tons'ofconcentrates per day.- The zinc concentrate feed contains 1.05% leadand31.4'% sulphur. As the ore progresses downwardly through the furnace,its temperature is increased so that on seven of the twelve hearths thetemperature is above'900 C., and on four of the hearths the temperatureis above 950 C. At the same time, a small amount of air is admitted inthe lower part of the roaster in order to partiallybu'rn theconcentrates to secure the necessary heat. The lead contentprogressively decreases as the ore passes downward'through the roaster.The ore (PDC) discharged from the roaster contains 0.027% lead and'-18.1%.sulphur.- 1 a In another run, the same species ofzincconcentrates are fed at the rate of 1 25 'tons'per day to theaforementioned multiple'hearth furnace; and the dischargedprodnotcontains 0.012% lead and 17.1% sulphur. 'Other runs, described morefully hereinafter, give similar results. a C I During some of theseruns, :it isfound convenientto burn "fuel gas on the fourth, tenth,:an'dthe twelfth hearths of the furnace in order better to control the.tem'-' perature distribution. With some concentrates fuel may be used asa convenience, but its use is not a necessity. In another run forexample, "122 tons per day of concentrate containing 0.55% lead are fedtov the multiple hearth furnace; and the discharged material contains0.020% lead. No fuel gas is burned during this run.

The elimination of cadmium by the method of this invention is also muchgreater than that of prior art processes. In the run first-mentionedhereinbefore, the cadmium content of the feed material is 0.10% whilethe cadmium content of the material discharged from the furnace is only0.027%. i

In another run, zinc sulphide concentrate is fed to the aforesaidmultiple hearth furnace at the rate of approximately 100 tons per day.The feed assays approximately 57% zinc, 0.55% lead, and 32% sulphur.Duringoperations, temperature of the second hearth from the top ismaintained at approximately 800 C., the third hearth .at 880 C., fourthto ninth hearths inclusive at 900 to 935 C., tenth hearth at 800 C.,eleventh hearth at 765 C., and the twelfth hearth at 525 C. Thefollowing results are obtained. For a 24-hour period when the amount ofconcentrate fed to the roaster is 93.9 tons, the product assays 0.007%lead and 19.5% sulphur. For a 24-hour period when the amount ofconcentrate fed to the roaster is 98.6- tons, the product assays 0.009%leadand 19.0% sulphur.

As mentioned in the first described run, when the furnace is fed with azincvsulphide concentrate assaying 55.0% zinc, 1.05% lead, and 31.4%sulphurat the rate of 132 tons per day, the product assays 0.027% leadnd 18.1% sulphur. When the furnace is fed with .zinc concentratesassaying 54.0% zinc, 2.08% lead, and 31.5% sulphur at the rate of abouttons per day, the product assayed 0.054% lead and 16.5% sulphur. Whilespecificruns have been described, it is not wished to limit thisinvention to the conditions described in the runs. In other runs, theinvention is practiced successfully using different temperatures thanthe ones recited hereinbefore. It will be readily understood that theconcentrate production of ,various mines will influence to aconsiderable extent the temperature and atmospheric conditions required,the more refractory ores or concentrates requiring temperature andreduction conditions of the higher order. In some of the runs,temperatures in excess of 950 .C. are used on two to five or more ofthehearths with good results. By means of preheated gasessuch .asderived from combustion of auxiliary fuel or'as derived from thesubsequent "desulphurizing step to be described, great leeway isprovided in exercising control both over temperatures and over sulphurcontent of the concentrates during passage through the furnace.

' Itisnecessary 'to remove the sulphur from the PDC in orderto prepare ahigh grade calcine suitable for ,agglomeration for'feeding'to reductionfurnaces. To this end, an autogenous desulphurizing step is carried outin a gas agitated bed operated under oxidizing conditions. The preferredpractice of this invention, therefore, calls for the passing of thepurified product (PDC) discharged from the multiplehearth furnace into aboiling bed reactor. Through tuyer'es in the Walls or floor of thisreactor, air'or other oxidizing gas is introduced in quantitiessufficientboth toagitate and to oxidize the particles of zinciferousmaterial. 'By means of this process, it is ,possible to desulphurizeautogenously purified .PDC. containing as little as 1.0%. sulphidesulphur.

The purified and desulphurized zinc calcine produced by the practicedisclosed herein issuitable after. agglomeration for making of .highpurity zincmetal-directfrom reduction furnaces without thenecessity'ofsubsequent redistilling or refining and for manufacture ofcommercial lead-free zinc-oxide pigments.

The invention will be described further with reference to thedrawingswherein:

Fig. 1 is a diagrammatic view of one form of apparatus in which theprocess of the invention is performed; and

'Fig. 2 is a digrammatic view of another form of apparatus'suitable forcarrying out the process of the invention.

Referring to the drawings, particularly to Fig. 1, represents a multiplehearth furnace and 11 a fluidized bed reactor. Zinc sulphide zincconcentrate feed, preferably dry, enters the multiple hearth furnace 10through seal 12 from suitable conveyor 13 through a bin 14. Part of exitgases from the fluidized bed reactor 11 may enter the multiple hearthfurnace directly or, after being divested of fume products in dustcollecting apparatus 15 may enter the multiple hearth furnace 10 throughduct 16 and be admitted to various hearths in desired quantities andpass upwardly through the furnace in counter-current flow relationshipto the downwardly moving zinc concentrates. The gases leave the multiplehearth furnace 10 through duct 17, pass through waste heat boiler 18,through fan 19, through a dust and fume separating device 20,which'suitably may be a Cottrell type electrostatic precipitator, andthrough duct 21 to further process- 1ng apparatus such as a contactsulphuric acid plant which is not shown. The shaft 22 of the multiplehearth furnace 10 and the rabble arms 23 attached thereto are rotatedata suitable speed such as l R. P. M. by drive umt 24. The rotatingrabbles cause the concentrates to feed across the hearths and downthrough the roaster in a manner well-known to those skilled in the art.The ore which has been treated in the multiple hearth furnace leavesthrough conveyor 25 and star valve 26 which discharges into surge bin27. From this bin pipe 28 delivers the deleaded concentrate to feeder 29which supplies deleaded sulphur-containing ore to fluidized bed reactor11. Feeder 29 may suitably be a screw conveyor driven by a variablespeed drive 30. The material delivered by feeder 29 to reactor 11 ismaintained in a fluidized state in the bed 31 by means of gas such asair blown in through tuyeres 32 located preferably in the floor of thereactor 11. Air is supplied to tuyeres 32 from a suitable supply such asa blower 33. An amount of air is supplied which will give a linearupward gas velocity in reactor of between one and three feet per second.Gases leaving reactor 11 pass through duct 34 to enter dust separationdevice 15, suitably a refractory-lined cyclone. Part of the gas leavingthe cyclone 15 through exit duct 35 is diverted to the lower portion ofthe multiple hearth furnace through duct 16 as described hereinbefore,while the remainder of the gas passes through duct 36 to waste heatboiler 37 thence through fan 38, Cottrell 39 and on through pipe 40 to afurther processing apparatus such as a sulphuric acid plant which is notshown.

Overflow of oxidized calcine from the fluidized bed 31 passes through anoverflow port 41 and seal 42 through conveyor 43 to a bin 44 and thenceby way of conveyor 45 to the next processing step. Dust separated incyclone 15 and in waste heat boiler 37 may join through conveyors 46 and47 the overflow product delivered into bin 44. Dust and fume recoveredby Cottrell 39 pass by conveyor 48 to a further processing step.Alternatively, the Cottrell dust from conveyor 48 may be diverted to thebin 44 or may be returned, if desired, to raw concentrate feed enteringfurnace 10 through conveyor 13.

Dust recovered in waste heat boiler 18 passes through conveyor 49 to thenext processing step or suitably may be returned to multiple hearthfurnace 10 through conveyor 13. Leady fume collected by Cottrell passesby conveyor 50 to the next processing step.

In another variation of the invention, referring to Fig. 1, exit gasesfrom the reactor 11 are passed directly mto multiple hearth furnace10.It is also possible to discharge PDC from the bottom hearth of furnace10 directly into reactor 11.

In some circumstances, it is not necessary to place a dust catcher 15ahead of waste heat boiler 37. Also,

8 it is not always necessary to pass multiple hearth furnace exit gasesthrough waste heat boiler 18, for example when operating with a cold topfurnace. For purposes of illustration, however, there has been shown amore general arrangement of our invention.

In the operation of this process, it is convenient to vary theproportion of fluidized reactor exit gases diverted through duct 16 tomultiple hearth furnace 10 by varying the speed of either or both fans19 and 38. Enough gas is diverted through furnace 10 to develop andmaintain temperatures suitable for eflicient deleading. Experiments haveshown that it is desirable to maintain temperatures in excess of 900 C.on at least two of the hearths of furnace 10 and preferably to maintaintemperatures in excess of 950 C. on at least five hearths. Theconcentrate feed rate through the system is adjusted so that with an airvelocity through reactor 11 of one to three feet per second there willbe a suitable stoichiometric excess of oxygen in the exit gases passingthrough duct 34. In general, it is desirable to maintain an oxygenconcentration of 1% to 7% in these gases.

In another of the preferred methods, as the first step in purificationof sulphidic zinc'concentrates, the concentrates are fed continuouslythrough a first fluid bed reactor and thereafter through a second fluidbed reactor.

Fig. 2 illustrates this arrangement for carrying out the invention. Inpurifying reactor 51 a bed of zinc concentrates is maintained in anagitated condition by injection of air through tuyeres 53 located in thefloor of the reactor. Air is supplied from any convenient source such asblower 54. Zinc concentrates, preferably thoroughly dry, are deliveredfrom a stockpile or other facilities, not shown, by conveyor 55 to feedbin 56. A conveyor 57, which suitably may include a weighing device,delivers concentrate to reactor feeder 58 which may suit-. ably be ascrew conveyor driven by variable speed drive 59.

Temperature of the reactor 51 may be controlled by varying the ratiobetween air input rate and concentrate feed rate. In general, it isfound preferable to maintain a fixed air input rate and to vary theconcentrate feed rate, for example by means of variable speed drive 59,to control the temperature within the reactor bed 52.

It is particularly convenient to control the bed temperatureautomatically by means of a conventional temperature recorder andcontroller which, through suitable linkage, adjusts the concentrate feedrate in response to variations indicated by a thermocouple in theboiling bed. A thermocouple may be located 6" to 18" above the bottom ofthe bed; but because of the remarkable uniformity of temperature in aproperly operated fluidized bed, location of thermocouple is not at allcritical. 7

It is the preferred practice to maintain as high a bed temperature aspossible without incipient fusion of the particles in the bed. If toohigh a temperature is reached, the bed may sinter and lose its mobilitycharacteristics. The highest workable temperature depends upon thespecies of concentrates being treated as some compositions fuse at lowertemperatures than others. In general, it has been found possible tooperate consistently with bed temperatures of the order of 1000 C. It isalso interesting to note that in comparative tests in which the samespecies of zinc concentrates was treated in a multiple hearth furnaceand in the mobile bed reactor,

the operating temperature attainable before sintering or I agglomerationoccurred was significantly higher in the latter reactor.

Gaseous reaction products together withthe sublimed impurities andsmaller particle size constituents of the concentrate feed leavethe topof the reactor 51 and flow through short duct 60 to a dust separatingdevice 61 which may conveniently be a cyclone separator. Alternativelythe cyclone can be located within the reactor or in an upward extensionthereof. However, with presently available materials of construction, itis usually -'9 more convenient to locate the deduster exteriorly to, butcontiguous with, the reactor.

Exit gases leave cyclone 61 through a duct 62 and pass successivelythrough a waste heat boiler 63, a fan 64, a fume-separating device 65,which suitably may be a Cottrell electrostatic precipitator, and onthrough a duct '66 to further processing apparatus, not shown, such as acontact sulfuric acid ,plant. Dust settled in waste heat boiler 63 iswithdrawn through pipe 67 and delivered to bin '68 or alternatively isreturned to feed bin 56 by suitable conveyingmeans, not shown.Leady'fume and other impurities volatilized by the reactor are recoveredin Cottrell 65, withdrawnby conveyor 69, and are passed to a.-furtherprocessing operation for recovery of the contained metal values.

Dust caught in the cyclone 61 passes through downspout 70 to bin 71which may'deliver'through pipe 72 to a feed bin 73 or the dust may'bereturned to reactor 51 as described hereinafter. The nature of thecyclone dust depends on the mode of operation of the reactor and thetemperatures of the gas .in the rdedus'ter, as will be disclosedhereinafter. In general, the greater the linear gas flow rate upwardthrough the reactor and the smaller the size of the concentrateparticles, 'the more will be the amount of dust carried over with theexit gases. Conversely, lower air rates and larger particle sizes tendto lessen the amount of dust in the exit gases. There is a lower limitto the gas flow rate below which the boiling bed tends to lose its"mobility characteristics. In general, gas flow rates of l t'o'-3 feetper second maintain satisfactory mobility of the bed. Particle size andparticle size range vary considerably from one species of concenrate toanother. Even-fairly'large size particles, such aspellets formed bytherolling-actionof concentratesin-storageor-in passing through a dryeror tumbling action conveyor, will fluidize satisfactorily if the sizerange is not too narrow.

If the exit gas from reactor 51 is allowed to chill before or when itenters the deduster 61, there is a tendency for some of the sublimed.and volatile compounds to come down with the dust. If it is desired toblend this dust with the purified concentrates which overflow from thereactor 51, then thegases are kept hot until they leave the .deduster.duce .a lead-enriched product as well as a lead-impoverishedmateriaLitis convenient to chill the gases before they enteror while they .passthrough-the deduster. For thislatterpurpose the deduster may'take theform of a waste heat boiler followed by a cyclone. Under somecircumstances it is desirable to recireulate a part of the cyclone dustfrom the bin 71 back through the reactor 51. For this purpose the dustis passed throughpipe 74 to join with the concentrate at feeder 58through bin 56, or it can enter the reactor51 through a similar butseparate feeder, not shown.

As indicated above, one of the methods of practicing this inventioncontemplates chilling the exit gases leaving reactor 51, by passing'saidgases first through a waste heat boiler and thereafter'through a'cyclonein the reverse order o f'that shovvn'in Fig. 2. By operating in thismanner it ispossible'to producea lead-enriched product suitable, afterdesulphurization, for the production of prime western zinc metal, leadedZinc pigments, and so forth. For this mode of beneficiation, cyclonedust frombin 71 and boiler-dust from bin 68 are comingled anddesulphurized in afluidized bed reactor not shown which, in effect,operatesin parallel with reactor75 and is essentially .a duplicateof'theiequipment assoc'iated'therewith.

-Aswconcentrate enters through feeder '58, PDC is withdrawn from.thereactor .51. Thismay be done in any convenient mannerone especially.convenient way being that there is little danger of short circuitingbetween feed and overflow. Whether the product is withdrawn from thesurface-of the bed, from the bottom, or from an intermediate level, orWhether the feed be introduced above, at, or beneath the top of thebed,-seems to make little ditference; the overflow product is notcontaminated with feed material.

From 'bin 73 PDC is withdrawn at regulated rate through conduit 79 byfeeding device 80, suitably a multiple screw conveyor driven by variablespeed drive 81. Feeder 8t) discharges PDC into reactor 75 where it formsthe fluidized bed 82. Air for agitation and oxidation of boiling bed 82is supplied by suitable means such as blower 83 through tuyeres 84suitably disposed in the bottom or lower'walls of the reactor 75. Anamount of air is supplied which will give an upward gas velocity inreactor 75 or" the-order'of l to 3 feet per second. Gases leave'reactor75 through .shortduct 85 and enter dust separation device 86, whichsuitably may be a'refractorylined cyclone or acyclone lined withcorrosion-resistant steel :of the 28% chromium variety. Gases pass fromdust separation device 86 through duct 87 to waste heat boiler'fifi,thence through fan "89 and Cottrell 90 and on through pipe 91 to furtherprocessing operation such as a On the other hand, if it is desired to.pro-

sulfuric acid plant, not shown. Prune collected by Cot- =trell passes byconvey0r 92 to asubsequent processing step. Dust recovered in waste heatboiler 88 passes by conveyor 92 to join the dust'dis'charged fromcollection device 86 in bin 93.

Overflow from boiling bed 82 0f reactor 75 passes through pipe95, starvalve 96 and pipe 97 to bin 93. This overflow. product, together withthe dust recovered from cyclone-86 and waste heat boiler 88, constitutesthe final purified oxidized zinc calcine product of this process.

Further description of this invention is given in the following example.In this example which pertains to the first orpurificationstep, thereactor consists of a steel shell 256 high by 4"1l diameter lined with 4/2" of insulatingbrick-and 4%" of firebrick up to 7'3 level and with 4/2" of firebrick above this point. To conserve heat the external surfaceof the reactor shell is lagged with insulationsuflicient to raise'thesteel temperature to 200- 250 C. Reactor gases pass through a cyclone,thence through aflueand variable speed fan to a contact sulfuric acidplant gas 'purification system. Dry zinc concentrates enter the reactorat about the five-foot level and are fed by a 4 diameter screw conveyorwhich in turn is supplied by a feeder of the constant weighttype-equipped with a variable speed drive fforready feed weightadjustment. A star gate arrangement seal-s the entry to the screwfeeder.

Air-is supplied -.to the reactor'through a number of tuyeres insertedthrough the refractory floor of the reactor or alternatively throughspokes 'of various lengths inserted horizontally through'the wall of thereactor at floor top level.

Reactor 'bed overflow .ports are provided at various levels, but for theresults-reported here the overflow ports at S-foot and 3 /2-footelevations are used.

To start a reactor of this type it is usually suflicient-to preheat thebrickwork to abright'cherry red temperature, turn on the air and startthe concentrate feed. Some species of oncentrate ignite more readilythan others. Sometimes it is convenient to light OE With a readily'burnable high iron concentrate and then switch to the less readilycombustible material. On occasion it has also been found helpful topreheat the air before it enters the reactor.

Temperature of the-boiling bed'is measured with thermocouples insertedthrough the reactor wall. In a properly Operating bed there are no morethan minor temperature differences fromside to side and from top tobottom of the bed. Temperatureis controlled 'by rate of feed addition.With dry concentrates and unpreheated air it is entirelypractical tomaintain a sulphide sulphur 11 concentration in the bed of 24% with a32% sulphur feed and to operate at 1000-1100 C.

In Table l are presented data typifying the operation of theabove-described reactor when purifying zinc sul- 12 Concentrate feedenters the highest reactor in the series and passes downward throughappropriately disposed overflow conduits to the gas agitated bed below.While theoretically any number of stagesmay be placed in phideconcentrates. In runs 17A through D the gases series flow asdescribed,'practically it is considered that emerging from the boilingbed are chilled before and durthree such stages are the workable limitin the present ing their passage through the cyclone deduster. In runsstate of the art, especially if bed temperatures of the 19, 20A, and20B, an attempt is made to conserve the order of .1000 C. are employed.heat in the gases between the time they leave the bed and In anotherarrangement it is found desirable to comthe time they leave the cyclone.Heat conservation is bine in stagewise relationship a boiling bedpurification accomplished by lagging the upper part of the reactorsection and a boiling bed desulphurizing section. The with heatinsulation material and by using a refractoryoperation of the boilingbed desulphurizer is the same lined cyclone. With bed temperature of1050 C., the as that previously described for the purifier except thattemperature of the gases leaving the cyclone is 950 C. an excess of airis employed in the desulphurizer whereas Table 1 Run No. 17A 17B 170 17D19 20A 20B Bed Depth, feet 5 5 5 3.5 3. 5 3. 5 Feed Rate, Tons/Day 10.412.5 17.4 14.5 18.4 21.0 21.8 Airflow Rate, C. F. M. STP- 200. 220 222225 225 22s 2 Bed Temperature, C 1, 050 1,075 1,000 1,075 1,050 1, 050,050 Percent of Product Appearing in Bed Overfl0w 64. 1 62.3 61. 2 64.67.6 67. 1 59.2 Percent of Product Appearing in Cyclone Heavies 22.927.9 27.7 18.0 13.4 8.6 13.8 Percent of Product Appearing in CycloneLights 13.0 9. 8 11. 1 17.4 19.0 24.3 27.0 Lead Content of Feed,percent 1. 13 1.13 1. 45 0. 85 0.35 0. 36 0. 61 Lead Content of BedOverflow, percent 0.04 0. 07 0. 028 0.016 0.016 0.024 Lead Content ofCyclone Heavies, percent. 1. 61 2. 1.36 0. 24 0.21 0. 60' Lead Contentof Cyclone Lights, percent 4. 26 8. 02 7. 50 3. 75 2.07 1.41 2.00Percent of Lead in Feed Appearing in Bed Overflow 3.8 1.9 1.9 1.9 2.92.9 2.2 Percent of Lead in Feed Appearing in Cyclone Heavies 51.0 36.741.5 26.5 10.1 4.8 13.2 Percent of Lead in ig ts 45.2 61.4 57.6 71.687.0 92.3 84-6 Other modes of practicing the invention are described astoichiometric deficiency of air, with respect to the rate hereinafter.of input of metallic sulphides, is maintained in the puri- It has beenfound that, for a given set of conditions, fication step. Temperaturecontrol in the desulphurizer the higher the sulphide sulphur content ofthe bed of a can be achieved by varying the quantity of excess air orfirst or purifying reactor, the greater is the degree of by introducinginto the bed a vaporizable coolant such volatilization of impurities. Inthe simplified version of as water or drip liquors from nearby acidplant or by the invention described -above--and which is entirely m ansf Water Steam'generating tubes Suitably P adequate for mostapplicationsheat and temperature 40 Automatic temperature control can beachieved by reguof the bed are developed by burning a portion of thelating the addition of coolant by means well-known in sulphides in thebed. The amount of sulphide thus rethe art. quired to be burned can besubstantially lessened by pre- In desulphurizing in a gas agitated bedwe have found heating the incoming air and by preheating the incoming itnot only possible but entirely practical to operate with zinc sulphideconcentrates. For example, with air preonly a few percent stoichiometricexcess of air and yet heated to 800 C. and concentrate feed preheated toobtain calcine running consistently less than 1% total 500 C., theattainable sulphide sulphur content of the sulphur content. The gas fromsuch a reactor contains bed rises from about 24%, when operated underthe conapproximately 12-14% S0 It is convenient to reguditionspreviously described, to about 29%. late the concentrate feed rateautomatically by means Air preheating is conventionally accomplished byreof an oxygen analyzer such as the Beckman Oxygen cuperative orregenerative means such as shell and tube Analyzer and controller. Thisdevice continuously samheat exchanger or a pebble heater. We have found,howples and determines the oxygen content of the gases leavever, that amuch simpler and cheaper way of preheating ing the gas agitateddesulphurizing bed and in turn air consists of burning the fuel, such asnatural gas, adjusts the concentrate feed rate to maintain the oxygenunder pressure right in the air duct (brick-lined) ahead content of theexit gases at a predetermined value. Good of the tuyeres. The hotter themixture of air and prodresults are obtained even when the oxygen contentis ucts of combustion, the less sulphide sulphur needs to be held as lowas 1% O in the exit gases. burned in the bed fluidized by these gases.When superimposing a purifying bed furnace on a In certain applicationsall or a part of the heat redesulphurizing bed furnace, it is necessaryto adjust the quired in the purification step is supplied by heattransgas flow rates to give a linear flow rate in each of the fersurfaces suitably disposed in or around the boiling reactors in seriesof about 1 to 3 feet per second. bed. Such heat transfer surfaces mayconsist of refrac- Again using lead as an example, it has been observedtory or alloy tubes through the interior of which pass that the leadcontent of the solids which pass out of hot fluid, for example, flameand Products of combusthe purifying reactor with the gases tends to beassotion from gas burner positioned at lower end of tube. i d ith thaller particles. This circumstance Tubes or other heat transfer surfacesmay also be heated makes i ibl t on entrate the lead by recycling the bywithin contained electrical resistance elements. Alcyckme, boiler, d fldusts through the d l i ternatively, molten salts or molten metal oralloy at suitactor or reactors d assing the gases through an elecabletemperature In11y be circulated through the tubestrostatic precipitator(Cottrell) for recovery of lead Further enhancement of p y eliminationmay be fume. For an extreme degree of lead concentration a attained yStageWiSe arrangement of boiling beds- I11 multiple stage Cottrell isused. Material collected in the such an arrangement, exit gases from thelowermost d first stage is recycled through the deleaders while ahigh Pthrough tllytireS in a refractory septum P g lead product is recoveredfrom the second and third the lowermost reaction chamber from the oneimmediatestages of the precipitator. 1y above it and fluidize the solidsin the higher reactor. The gas ag tated bed desulphurizer and oxidizer1s a to a high of 24 to 26% S.

highly desirable component of this invention if it is to be practiced inits most economical form. Other desulphurization methods suffer majordisadvantages by comparison. Sulphide sulphur content of PDC from thepurification step ordinarily may vary from a low of 10% To dead roastthis material in a multiple hearth roaster requires the combustion oflarge amounts of auxiliary fuel such as natural gas. The sulphur dioxidecontent of gas from such a roaster is of the order of 6 to 7% S byvolume. Desulphurization of PDC in a suspension roaster also requiresthe use of substantial amounts of auxiliary fuel. The exit gas from thisroaster would contain about 7 to 9% S0 by volume.

By carrying out the desulphurization step in a gas agitated bed, it ispossible to roast PDC entirely autogenously and to produce an exit gasof the order of 12% S0 by volume. Compression of the S0 into so muchsmaller volume yields important economies in the operation of thesubsequent gas purification equipment and catalyst system of the acidplant. S0 the volume of gas required to make one ton of H SO is about128,000 cubic feet (at 95% conversion eificiency in the acid plant);whereas, at 12% S0 the volume required for one ton of H 50 is only64,000 cubic feet.

Compared with suspension roasting, a gas agitated bed desulphurizer asdisclosed in this invention, is relatively non-sensitive with respect toparticle size of the PDC to be roasted. With a suspension roaster, onthe other hand, it is necessary to comminute the feed to a high degreeto insure satisfactory operation even with the use of auxiliary fuel.

When operated as a finish desulphurizer, the fluidized bed reactor isnot efficient as a purifier. By the methods disclosed in this invention,it is now possible to accomplish the substantially complete eliminationof those impurities which under controlled conditions of temperature andatmosphere will sublime or become volatile, such as compounds of lead,tin, cadmium, and the like. This For example, at 6% invention providesmeans to accomplish both purification and desulphurization in aneconomical and commercially practicable manner. By doing thepurification before the desulphurization We are able to utilize the gasagitated bed method With all its advantages; whereas, if, for example,non-deleaded concentrates were fed to the fluidized bed desulphurizer,only a lead-containing calcine Would be produced.

We claim:

1. The process for purifying and desulphurizing zinc sulphide orecontaining volatilizable compounds of metallic impurities such as lead,tin and cadmium which comprises passing an oxidizing gas in'contact withthe ore in a first oxidizing zone at a rate regulated to oxidize atleast 10% and not more than of the sulphite content of the ore and tomaintain the temperature in at least a substantial portion of said firstzone in the range of 850" C. to 1050 C., and thereafter completing theoxidation of the ore by transferring the partially oxidized ore to asecond oxidizing zone through which an oxidizing gas is blown tomaintain a bed of the ore in a fluidized state and to oxidizeautogenously the remaining sulphide content of the ore.

2. The process as defined in claim 1 wherein said first zone comprises aplurality of interconnected, vertically superposed, horizontal sub-zonesand said ore and gas are passed therethrough in countercurrent relation.

3. The process as defined in claim 1 wherein, in said first zone, saidgas is passed upwardly through a bed of said ore at a velocity tomaintain the bed in a fluidized state.

References Cited in the file of this patent UNITED STATES PATENTS735,903 Picher Aug. 11, 1903 1,992,049 Young Feb. 19, 1935 2,035,699Fowler Mar. 31, 1936 2,120,475 Stimmel et al June 14, 1938 2,650,159Tarr et a1 Aug. 25, 1953 2,689,176 Klepetko et al Sept. 14, 1954 UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 2 847, 294August 12, 1958 Carleton 0, Long et al.

It is herebi certified that error appears in the-printed specificationof the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below.

Column 14, line 15, for "sulphite" read sulphide a Signed and sealedthis 21st day of October 1958.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner ofPatents

1. THE PROCESS FOR PURIFYING AND DESULPHURIZING ZINE SULPHIDE ORECONTAINING VOLATILIZABLE COMPOUNDS OF METALLIC IMPURITIES SUCH AS LEAD,TIN AND CADMIUM WHICH COMPRISES PASSING AN OXIDIZING GAS IN CONTACT WITHTHE ORE IN A FIRST OXIDIZING ZONE AT A RATE REGULATED TO OXIDIZE ATLEAST 10% AND NOT MORE THAN 70% OF THE SULPHITE CONTENT OF THE ORE ANDTO MAINTAIN THE TEMPERATURE IN AT LEAST A SUBSTANTIAL PORTION OF SAIDFIRST ZONE IN THE RANGE OF 850*C. TO 1050*C., AND THEREAFTER COMPLETINGTHE OXIDATION OF THE ORE BY TRANSFERRING THE PARTIALLY OXIDIZED ORE TO ASECOND OXIDIZING ZONE THROUGH WHICH AN OXIDIZING GAS IS BLOWN TOMAINTAIN A BED OF THE ORE IN A FLUIDIZED STATE AND TO OXIDIZEAUTOGENOUSLY THE REMAINING SULPHIDE CONTENT OF THE ORE.